1. Field of the Invention
The present invention relates to a charged particle accelerator that prevents electrical discharge.
2. Description of the Related Art
An accelerating tube has a structure in which electrodes (accelerating electrodes) of multi-stages are stacked and voltage of several tens of kV is applied between the accelerating electrodes. Thus, in some cases, dust, residual gas, charging and the like cause electric discharge between the stacked electrodes. Generation of electric discharge temporarily reduces voltage and results in unstable applied voltage. Particularly, under a vacuum condition, an electron is emitted from an electrode with a low potential and collides with an electrode with a high potential, so that metal of the electrode is evaporated in vacuum to degrade insulation properties between the electrodes, thereby leading to large-scale electric discharge. Generation of electric discharge makes it impossible to apply voltage to the accelerating tube, so that the entire system is stopped. Japanese Unexamined Patent Application Publication No. Hei 6-203996 discloses that accelerating electrodes are used as magnets to generate lines of magnetic force around the electrodes so as to prevent electric discharge of the entire accelerating tube.